Xerographic Copying Apparatus

ABSTRACT

A xerographic copying apparatus wherein the development unit employs a bucket conveyor for transport of a mixture of toner and carrier particles from a magazine to an elevated point prior to cascading the particles onto a travelling xerographic surface. The surplus of the mixture is returned into the magazine. The particle-contacting surfaces of the magazine and of all other parts which move relative to the particles (or vice versa) consist of highly conductive material and are connected to the ground. The particle-contacting surfaces of the buckets and all other parts which move with the particles consist of insulating material. This allows for more accurate control of the overall charge of the mixture.

United States Patent [191 Mueller et al.

[ 1 Feb. 4, 1975 1 1 XEROGRAPHIC COPYING APPARATUS [73] Assignee: Agfa-Gevaert Aktiengesellschait,

Leverkusen, Germany 22 Filed: Nov.2l, 1972 [21] App]. No: 308,332

[30] Foreign Application Priority Data 3,641,981 2/1972 Hudson 118/637 3,663,291 5/1972 Royka 117/175 3,674,532 7/1972 Morse 117/175 3,682,538 8/1972 Cade et a1 355/3 Primary ExaminerMervin Stein Assistant Examiner-Leo Millstein Attorney, Agent, or Firm-Michael S. Striker [57] ABSTRACT A xerographic copying apparatus wherein the development unit employs a bucket conveyor for transport of a mixture of toner and carrier particles from a magazine to an elevated point prior to cascading the particles onto a travelling xerographic surface. The surplus of the mixture is returned into the magazine. The particle-contacting surfaces of the magazine and of all other parts which move relative to the particles (or vice versa) consist of highly conductive material and are connected to the ground. The particle-contacting surfaces of the buckets and all other parts which move with the particles consist of insulating material. This allows for more accurate control of the overall charge of the mixture.

5 Claims, 1 Drawing Figure 1 XEROGRAPHIC COPYING APPARATUS BACKGROUND OF THE INVENTION The present invention relates to xerographic copying apparatus in general, and more particularly to improvements in xerographic copying apparatus of the type wherein a mixture of toner particles and carrier particles is cascaded over a moving xerographic surface to convert electrostatic latent images into powder image patterns. Still more particularly, the invention relates to improvements in xerographic copying apparatus wherein the means for supplying the toner-carrier mixture preferably includes a bucket type conveyor and wherein the electrostatic charging of toner-carrier mixture takes place as a result of triboelectric effects during circulation of the mixture at the development station.

In the just described types of xerographic copying apparatus, the mixture of toner and carrier particles is charged due to triboelectric interaction between the particles on the one hand and as a result of interaction between the particles and the particle-contacting surfaces of component parts at the development station on the other hand. Such widely different interactions almost invariably cause the particles of the toner-carrier mixture to accumulate a charge which is different from the charge of the entire mixture.

In order to insure contrasting development of a latent image, i.e.. to insure the transfer ofa sufficient quantity of toner particles onto the image areas and the absence of toner particles on non-image areas of the xerographic surface, it is necessary to control the charges in such a way that the ratio of the charge of latent image to the charge of toner particles and the carrier particles remain within a very narrow range. As a rule, the particles of toner tend to cling to non-image areas of the xerographic surface due to the presence of adhesive forces, such as residual charge on non-image areas.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved xerographic copying apparatus wherein the triboelectric charging of the toner-carrier mixture is more homogeneous than in presently known apparatus so that the powder images which are produced in the improved apparatus exhibit a highly satisfactory contrast and the non-image areas of the xerographic surface remain free of toner.

The invention is embodied in a xerographic copying apparatus wherein a xerographic surface is arranged to carry electrostatic latent images along a predetermined path. The copying apparatus includes a development unit which serves to convert latent images on the xerographic surface into powder image patterns and comprises a magazine for a supply of intermixed toner and carrier particles, and a conveyor which is arranged to withdraw intermixed particles from the supply in the magazine and to cascade the thus withdrawn particles over latent images on the xerographic surface whereby the surplus of particles reenters the magazine by way of a chute or analogous guide means and the particles become electrostatically charged as a result of triboelec trification during circulation in and back to the magazine.

In accordance with a feature of the invention, the development unit has a plurality of particle-contacting surfaces (including the surfaces of the magazine and of the conveyor) which exhibit neutral characteristics with respect to the overall triboelectric charge of the supply of particles.

The aforementioned plurality of particle-contacting surfaces include a first group of surfaces which move with the particles (the surfaces of this first group include the particle-contacting surfaces of the conveyor) and a second group of surfaces which move relative to the particles or vice versa (the surfaces of this second group include the surface of the magazine, the surfaces of one or more chutes or analogous guide means for the particles and/0r means for agitating the particles in the magazine). The surfaces of the first group preferably consist of insulating material and the surfaces of the second group preferably consist of a material which is a good conductor of electric current. The surfaces of the second group are connected to the ground.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved xerographic copying apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE is a fragmentary vertical sectional view of a xerographic copying apparatus which embodies the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawing illustrates the development unit in a xerographic copying apparatus wherein the mixture of toner particles and carrier particles is cascaded over the endless xerographic surface 7 of a rotary drum 8 by a bucket type conveyor 3. The buckets 5 of the conveyor 3 are filled with toner-carrier mixture during travel through a magazine 2 which contains a supply I of such mixture. The other stations of the development apparatus are of conventional design and, therefore, are not shown in the drawing. They may include a charging, an exposure, a transfer and a cleaning or discharge station of the type disclosed. for example, in the commonly owned copending application Ser. No. l8l,374, filed Sept. 17, 1971 by Szostak et al.

The mixture I is electrostatically charged in the magazine 2 by effecting a relative movement between its particles. The filled buckets 5 of the conveyor 3 travel from the lower level of the magazine 2 to a higher level, as at 4, where their contents are dumped onto a suitably inclined guide member or chute 6 for transport onto successive increments of the travelling xerographic surface 7. The discharge end of the chute 6 is located downstream of the aforementioned exposure station so that the material which is discharged by successive buckets 5 onto the chute 6 is caused to come in contact with the latent image on the surface 7. The mixture' of toner and carrier particles trickles along the surface 7 and thereby converts the latent image into a powder image pattern which is thereupon transferred onto discrete sheets or onto a continuous sheet at the aforementioned transfer station in a manner not forming part of the present invention.

The surplus toner-carrier mixture is intercepted by a guide member or chute 9 and is returned into the magazine 2 for recirculation by the conveyor 3.

ln accordance with a feature of the invention, the magazine 2 (or at least that portion of the magazine which is contacted by the particles of the supply I), at least the particle-contacting portion of the chute 6 and at least the particle-contacting portion of the intercept ing chute 9 consist of highly current-conducting material and are connected to the ground as indicated at 10. This enhances the uniformity of charges of the particles forming the supply I because, though some ofthe particles lose their charges on contact with the part 2,6 and- /or 9, they are recharged as soon as they reenter the mass of particles in the magazine. Since a particle which is about to be entrained by a bucket of the conveyor invariably passes through the mass of particles in the supply 1, this automatically insures that such particle accumulates a predetermined charge prior to contacting the xerographic surface 7. In other words, the charges of all particles in a bucket 5 which travels toward the point 4 of discharge is uniform. The buckets 5 (or at least those portions of the buckets which come into contact with the particles) consist of nonconducting material. Consequently, the particles which come into actual contact with the buckets 5 retain their charges during transport toward the discharge point 4.

The conductivity of particle-contacting portions of or entire members 6 and 9 is of advantage because the negligible discharge which takes place due to connection of members 6 and 9 with the ground is less damaging than a further charging of particles which could take place due to triboelectric interaction between the particles and the members 6 and/or 9 if the latter were not connected to ground and would not consist of conductive material.

The carrier particles of the supply 1 may constitute beads of glass with a coating and the particles of toner may consist of polystyrene or polyvinyl butyrol and a suitable pigment.

The walls of the members 2, 6 and 9 may consist of a metallic material having a specific surface conductivity in the range of lO Ohm". Alternatively, these walls may consist of brass or steel having a galvanically applied particle-contacting coating of chrome or nickel.

The buckets 5 of the conveyor 3 may consist of brass or steel with a coating of insulating material, such as a suitable lacquer. The coating of lacquer can be applied by electrophoresis to exhibit good mechanical and insulating characteristics. Alternatively, the buckets 5 may consist of a suitable synthetic plastic material which is a poor conductor of electric current.

It will be noted that the particle-contacting surfaces of the members 2, 5, 6 and 9 exhibit neutral characteristics with respect to the triboelectric charge of the overall mass of the mixture of carrier and toner particles. This is achieved by producing the members 2, 6 and 9 of highly current-conducting material and by connecting them with the ground. and by providing the conveyor 3 with non-conducting particle-contacting surfaces. At the same time, the thus constructed and connected members 2, 6 and 9 insure that the homogeneousness of the charge of particles forming the mixture of carrier and toner is not adversely affected by charging of particles as a result of frictional engagement with the surfaces of these members. The

grounded surfaces of the members 2, 6 and 9 will cause those particles which come into actual contact therewith to lose their charges. However, and as pointed out above, such particles are charged again as soon as they are moved out of contact with the surface of the member 2, 6 or 9 as a result of friction with other particles in the toner-carrier mixture. The ultimate result is a greater homogeneousness of the charges of particles in the mixture, i.e., a homogeneousness which is much more satisfactory than that which is achievable when the particles can increase their charges due to frictional contact with the surface of the member 2, 6 or 9. This also prevents excessive charging of particles. It is well known that, if a bead of carrier material is allowed to accumulate an excessive charge, it will attract a large number of toner particles with the result that the quality of the developed image is adversely affected by excessive toner concentration. An overcharged bead of carrier material is incapable of releasing sufficient numbers of toner particles on image-bearing areas of the xerographic surface (undertoning of the image) or of attracting toner particles from the non-image areas of the xerographic surface. In the apparatus of the present invention, the charge of particles forming the supply depends exclusively on the triboelectric characteristics of selected toner and carrier particles so that the charge can be determined and controlled with a high degree of accuracy. This insures the formation of powder images exhibiting a high contrast between image-bearing and non'image areas and being free of socalled grey spots on non-image areas.

If a bead of carrier material contains a relatively low charge, it cannot attract sufficient toner material for proper coating of image-bearing areas of the latent image or for attracting toner particles which tend to adhere to the non-image areas. Thus, insufficiently charged carrier beads will cause the formation of powder images with unsatisfactory contrast between imagebearing and non-image areas and with grey non-image areas. In the apparatus of the present invention, unsatisfactory charging of carrier beads is prevented by making the particle-contacting surfaces of the conveyor 3 of insulating material. In other words, the surfaces of parts (2,6,9) which move relative to the particles of toner and carrier material (or vice versa) conduct current and the surfaces of parts (buckets 5) which do not move relative to toner and carrier are made of insulating material. This insures that the charges cannot be dissipated and/or equalized even if the conveyor 3 is constructed and assembled to transport batches of mixture of toner and carrier beads along an elongated path, i.e., if the point 4 of discharge is remote from the supply l in the magazine 2.

The improved copying apparatus is susceptible of many additional modifications without departing from the spirit of the invention. For example, the chute 6 and/or 9 can be dispensed with or the ground can be connected with other stationary or movable parts which come into contact with particles of the supply 1 prior to transport of particles to the discharge point. Such other parts may include one or more elements of a device which is provided to agitate and/or mix the particles of carrier material and toner in the magazine 2. An agitating and/or mixing device is shown diagrammatically at ll.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can.

by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by letters Patent is set forth in the appended claims:

1. In a xerographic copying apparatus wherein a rotary drum-shaped xerographic surface is arranged to carry electrostatic latent images along a predetermined path, a development unit for converting said latent images into powder image patterns, comprising a magazine for a supply of intermixed toner and carrier particles; and a conveyor arranged to withdraw intermixed particles from said supply and to cascade the thus withdrawn particles over latent images in said path whereby the surplus of particles reenters said magazine and the particles become electrostatically charged as a result of triboelectrification during circulation in and back to said magazine, said conveyor including a series of discrete receptacles arranged to transport batches of intermixed particles from said magazine to a level above said supply and to dump the respective batches onto said xerographic surface upon reaching said level, said development unit having a plurality of particlecontacting surfaces including the particle-contacting surfaces of said magazine and said conveyor, said plurality of surfaces exhibiting neutral characteristics with respect to the overall triboelectric charge of said supply of particles and including a first group of surfaces which constitute the surfaces of said receptacles and move with said particles and a second group of surfaces. the particles which contact the surfaces of said first group being in motion relative to the surfaces of said second group, the surfaces of said first group consisting of insulating material and the surfaces of said second group consisting of current-conductin g material and being connected to the ground, each of said receptacles consisting of current-conducting material having a coat of insulating material so that the surfaces of said second group are provided on said coats, said second group of surfaces comprising a first chute arranged to convey particles discharged by said conveyor into said path and a second chute for intercepting surplus particles in said path and for returning the intercepted particles into said magazine.

2. A development unit as defined in claim l, wherein the surfaces of said second group consist of a metal whose specific surface conductivity approximates or equals 10""Ohm".

3. A development unit as defined in claim 1, wherein the surfaces of said second group consist of chrome.

4. A development unit as defined in claim 1, wherein the surfaces of said second group consist of nickel.

5. A development unit as defined in claim 1, wherein said coats consist of electrophoretically applied lacquer. 

1. In a xerographic copying apparatus wherein a rotary drumshaped xerographic surface is arranged to carry electrostatic latent images along a predetermined path, a development unit for converting said latent images into powder image patterns, comprising a magazine for a supply of intermixed toner and carrier particles; and a conveyor arranged to withdraw intermixed particles from said supply and to cascade the thus witHdrawn particles over latent images in said path whereby the surplus of particles reenters said magazine and the particles become electrostatically charged as a result of triboelectrification during circulation in and back to said magazine, said conveyor including a series of discrete receptacles arranged to transport batches of intermixed particles from said magazine to a level above said supply and to dump the respective batches onto said xerographic surface upon reaching said level, said development unit having a plurality of particle-contacting surfaces including the particle-contacting surfaces of said magazine and said conveyor, said plurality of surfaces exhibiting neutral characteristics with respect to the overall triboelectric charge of said supply of particles and including a first group of surfaces which constitute the surfaces of said receptacles and move with said particles and a second group of surfaces, the particles which contact the surfaces of said first group being in motion relative to the surfaces of said second group, the surfaces of said first group consisting of insulating material and the surfaces of said second group consisting of currentconducting material and being connected to the ground, each of said receptacles consisting of current-conducting material having a coat of insulating material so that the surfaces of said second group are provided on said coats, said second group of surfaces comprising a first chute arranged to convey particles discharged by said conveyor into said path and a second chute for intercepting surplus particles in said path and for returning the intercepted particles into said magazine.
 2. A development unit as defined in claim 1, wherein the surfaces of said second group consist of a metal whose specific surface conductivity approximates or equals 10 5Ohm
 1. 3. A development unit as defined in claim 1, wherein the surfaces of said second group consist of chrome.
 4. A development unit as defined in claim 1, wherein the surfaces of said second group consist of nickel.
 5. A development unit as defined in claim 1, wherein said coats consist of electrophoretically applied lacquer. 